Displays are found in numerous commercial and consumer devices. Because of various physical characteristics, flat panel displays tend to be favored over cathode ray tube (CRT) displays in many applications where size, weight and/or power consumption is of concern.
Flat panel displays, including e.g., liquid crystal display (LCD) devices come in many different sizes. Small LCD devices are used in applications ranging from calculators and wristwatches to point-of-sale terminals and gas pumps. Larger LCD devices are found in portable computers, desktop computer displays, and numerous other devices.
Known LCDs are frequently implemented as reflective, transmissive, or transflective devices. A reflective LCD, as the name implies, uses reflection to illuminate the display. FIG. 1 illustrates a known reflective LCD 102. The reflective LCD 102 includes a closed housing 109 which contains a liquid crystal cell 104 and a reflector 110. A screen 105 made of, e.g., glass, is used to seal the front of the closed housing 109. Light from an external light source 106 passes through the screen 105, liquid crystal cell 104 and is then reflected back towards the eye 108 by the reflector 110 located behind the liquid crystal cell 104. The liquid crystal cell includes, for example, front and rear polarizers with a layer of liquid crystal material sandwiched there between. The light absorptive characteristics of the liquid crystal cell are varied by changing an electric field applied to the layer of liquid crystal material. Thus, by varying an electric field images may be displayed on the LCD 102 and perceived by a viewer represented by the eye 108.
Reflective LCDs are generally the least expensive type of LCD and use the least amount of power. Reflective LCDs rely on ambient, e.g., external natural or artificial light sources for illumination. Accordingly, reflective LCDs do not include a backlight. Such displays operate satisfactory in well lit locations. However, because they lack an internal light source they are difficult to read in low light conditions which are often encountered indoors. For this reason, reflective displays have not found wide spread use in portable computers or other devices which may need to be used in low light conditions.
Transmissive LCDs such as transmissive LCD 103, illustrated in FIG. 2, use an internal light source 107, referred to as a backlight, for illumination. In the transmissive LCD 103, the backlight 107 is enclosed in an opaque housing 110 behind the liquid crystal cell 104 and display screen 105. Light from the backlight 107 passes through the liquid crystal cell 104 and display screen 105 before being perceived by a viewer, represented by the eye 108. Since the housing 110 is opaque, natural and/or ambient light from behind the housing is prevented from entering the liquid crystal cell from the rear of the housing.
Transmissive displays are well suited for use indoors under artificial lighting. For this reason, transmissive LCDs are frequently used in, e.g., portable computers and lab instruments. One drawback to transmissive displays is that they consume a relatively high amount of power due to the use of the backlight. In portable devices such as battery powered notebook computers, minimizing power consumption is important. Power consumption by the backlight is a major factor in determining the amount of time portable computers can be used between recharges.
Many portable computers include a brightness control which allows the intensity of the backlight used in a transmissive display to be manually adjusted by a system user. While manually adjusting the display brightness to the minimum setting which is acceptable to the user for a particular set of room conditions can maximize the time before the computer needs recharging, users are not accustomed to adjusting the brightness of their displays each time they move to a different room or ambient lighting conditions change. To allow for a transmissive display to be used in a wide range of conditions, the brightness of the display is normally set to a value which exceeds the brightness required for normal room conditions, e.g., so that the display can be used in higher than normal lighting conditions without having to adjust the brightness. Unfortunately, such intensity settings tend to waste power which, as discussed above, is a limited resource in the context of most portable devices.
Since the amount of power delivered by batteries is often a function of their size and thus weight, it is desirable to minimize power consumption requirements in portable devices to allow for longer periods of use between battery recharges and/or the use of smaller, lighter, batteries. It is desirable that any methods and apparatus directed to power conservation be at least partially automated so that a user need not make display adjustments each time lighting conditions change.
In addition to relatively high power consumption, another disadvantage of the known transmissive LCD 103 is that such displays are usually hard to read in direct sunlight. The difficulty in reading such displays in direct sunlight arises from the fact that incident sunlight reflected from the display screen 105 can be quite bright compared to the intensity of the light, originating from the backlight 107.
While some manufacturers of transmissive LCDs have incorporated high output backlight to enable out of doors use of transmissive displays, the relatively high power consumption of such devices renders them unsuitable for most battery powered applications.
Another type of known LCD device is the transflective LCD 111 illustrated in FIG. 3. Transflective LCD 111 combines features of the reflective and transmissive LCDs discussed above. As illustrated, a transflective LCD 111 includes a liquid crystal cell 104, partially transmissive reflector 116 and a backlight 107. The transflective display components are enclosed in an opaque housing 114 which is sealed in the front with a screen 105. Behind the screen 105 is the liquid crystal cell 104, transmissive reflector 116 and backlight 107. Because the housing 114 is opaque, it prevents external, e.g., natural or ambient light from entering from the rear of the housing.
In the transmissive display 111, the reflector is normally a white or silver translucent material that reflects some of the ambient light entering from the front, i.e., viewing side or surface, of the display 106 while still allowing light from the backlight 107 to pass through. Transflective LCD 111 is useful in a wide range of lighting conditions. For this reason, LCD 111 is frequently used where a display must function in both day and night light conditions, e.g., in gas station pump displays. Transflective displays suffer from some of the same power consumption problems, associated with the use of a backlight, discussed above in regard to transmissive displays. In addition, transflective displays tend to have relatively poor contrast ratios since partially transmissive reflector 116 must be partially transparent to let light from the backlight through.
In view of the above discussion it is apparent that there is a need for improved displays, e.g., LCD displays, which can be viewed easily in a wide range of light conditions. It is desirable that at least some of the new displays be capable of implementation without a backlight and the power consumption associated therewith.
From the above discussion, it is also apparent that there is a need for methods and apparatus directed to reducing the amount of power utilized by displays which incorporate backlights.